Effect of Time of Exposure on the Antimicrobial Potentials of Some Tropical Plants against Cocoa Pod Rot Pathogen- Phytophthora megakarya (B & G) in Nigeria

Abstract

Aims: To determine the effect of time of exposure on the antimicrobial effectiveness of some tropical plants in the control of black pod disease of cocoa caused by Phytophthora megakarya in Nigeria. Study Design: Completely Randomized Design (CRD). Place and Duration of Study: Plant Pathology Section, Cocoa Research Institute of Nigeria (CRIN), Ibadan, Nigeria, from 2015 to 2016. Methodology: The antimicrobial effects of aqueous extracts of six selected Tropical plants: Moringa oleifera, Sida acuta, Piper guineense, Ocimum gratissimum, Chromolaena odorata and Cymbopogon citratus against Phytophthora megakarya, causal agent of black pod disease of cocoa was investigated in an in vitro experiment. 200 g leaf samples of each of the plants used were surface sterilized, mashed separately and the resultant pastes were soaked in 200 ml of sterile distilled water and filtered. Varied concentrations of the extracts (10, 25, 50 and 75%) were pour-plated with freshly prepared potato dextrose agar (PDA). Disc culture (5 mm) of Phytophthora megakarya was inoculated into each of the PDA plates and incubated at 25°C for 144 hours. Results: Aqueous extracts of the six plants, at varied concentrations showed different levels of inhibition against the pathogen. C. odorata at all tested concentrations consistently reduced the mycelia growth and showed significantly highest percentage inhibition (P < .05) against the pathogen, followed by Sida acuta, P. guineense, O. gratissimum and C. citratus while M. oleifera gave significantly lowest inhibitory percentage (P < .05). Conclusion: Findings from this research showed that the antimicrobial effectiveness of all the botanicals used decreased with time contrary to that of the standard (chemical fungicide) used. Aqueous extracts of C. odorata having shown some promise, could therefore be used as bio-fungicide in the control of black pod disease of cocoa, but with intermittent application.